Precipitation-hardenable, aluminum-containing iron base alloy



ited States Patent PRECIPITATION-HARDENABLE, ALUMINUM- CONTAINING IRON BASE ALLOY William J. Buehler, Hyattsville, and Charles G. Dalrymple, Rockville, Md., assignors to the United States lorf America as represented by the Secretary of the The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to low aluminum content iron base alloys. More specifically the invention relates to low aluminum content iron base alloys containing no alloying metals in short supply but having superior strength combined with oxidation and corrosion resistance at elevated temperatures as well as acceptable static and dynamic mechanical properties at room temperature and to the process for producing these alloys.

The high temperature strength and oxidation resistance of the iron base alloys containing from 14 to 16% by weight of aluminum with 3 to 4% by weight of molybdenum have been known for some time. These alloys are, however, too brittle to permit satisfactory working at room temperatures. Iron base alloys containing 8 to 12% by weight of aluminum are sufiiciently ductile to permit such working but lack good oxidation resistance and strength at elevated temperatures.

The value of the aluminum addition to the iron base to prevent oxidation at elevated temperatures has been long recognized. The additionof the aluminum, however, promotes grain growth which interferes with working at room temperatures. Carbon present is believed to segregate at the grain boundaries upon cooling forming brittle carbides surrounding the iron-aluminum grains. These carbides cause brittleness and failures due to the rapid deterioration of the carbides. Increasing the aluminum content of the alloy merely decreases the ductility by aggravating the brittleness and large grain structure. The manufacture of carbon free iron-aluminum alloys in present industrial furnaces is practically impossible. U.S. Patent No. 2,172,023 to J. D. Gat and US. Patent No. 2,726,952 to E. R. Morgan suggest the addition of a small proportion of a metal such as titanium, molybdenum, tantalum, columbium and the like to combine with the carbon present to form uniformly distributed fine carbides of low solubility. In this manner the carbides are distributed about the mass to form nuclei of crystallization and are removed from the ironaluminum grain boundaries. In preparing these alloys the various metals are ordinarily melted, thoroughly commingled under conditions protecting them from excessive oxidation and, cast in the desired form. Alloys prepared in this manner have, however, been found to lack satisfactory strength and hardness at elevated temperatures.

In accordance with the present invention the disadvantages of the prior art are overcome and there is produced an iron-aluminum alloy which is sufificiently ductile to permit working at room temperatures and yet is capable of maintaining increased hardness and strength at elevated temperatures. This is accomplished by subjecting a specific iron base low aluminum content alloy containing a ternary metal to specific precipitation hardening treatments.

It is therefore an object of the. present invention to andother mechanical properties to permit the alloy to be worked at room temperatures but capable of being treated to give high strength and hardness at elevated temperatures.

A further object is to provide a process for preparing a low aluminum content iron base alloy capable ofsuperior strength and oxidation and corrosion resistance at elevated temperatures.

Other objects and the attendant advantages of the invention will become apparent to those skilled in the art as the invention is disclosed in the following detailed description.

The above objects are accomplished in accordance with the present invention by utilizing an iron-aluminum alloy system containing from about 8% to about 12% by weight of aluminum and adding to this alloy system from about 1% to about 3.5% of a third metallic element which promotes a precipitation hardening reaction. This ternary alloy system is subjected first to a solution treatment to produce an alloy of suitable ductility for working at room temperature. The alloy system is then subjected to an aging treatment to produce an alloy having the desired strength and oxidation and corrosion resistance at elevated temperatures.

The iron base alloy systems containing from about 8 to about 12% aluminum by weight have been found satisfactory in producing the alloy of the inventiomth'i's range providing suitable room temperature ductility and good oxidation resistance at elevated temperatures. However, the iron base alloy systems foundto have the most desirable characteristics are those containing about 8% by weight of aluminum.

The ternary metallic element which has been found to promote the significant precipitation hardening of the invention was titanium. From about 1% to about 3.5% of titanium must be present in the alloy system. Titanium percentages below about 1% produce too small'a change in hardness and are ineffective in causing the required precipitation to occur. Titanium percentages in excess of about 3.5 percent make the alloys brittle and render them useless as a structural material.

Carbon added to the alloy combinations was found useful in producing a refined grain size but contributes little to the precipitation hardening mechanism.

Metals other than titanium also do not contribute'sub'e stantially to this precipitation hardening mechanism.

The alloys of the invention may be melted by any suitable melting technique such as by electric are or induction methods. An atmosphere of hydrogen or helium may be used in the melting chamber to prevent excessive oxidation. Air atmosphere melting may beem ployed providing an inert gas blanket is placed over the melt or a suitable slag cover is placed on the molten alloy.

In accordance with the process of the invention the alloy is subjected to solution treatment by heating to a temperature between about 900 to about 1000 C. This temperature range is critical since temperatures above 1000" C. weaken the material by causing excessive grain growth. Temperatures below 900 C. are too low to place the alloy in the single phase solution range required for precipitation hardening. The material must be held at this temperature until a single phase solution is achieved. The time the alloy must be held at this temperature varies greatly-with the thickness and'mass of the material being heated and must be determined empirically. For an 0.1 thick specimen an optimum time for solution treatment of the alloy of the invention has been found to be about one hour or less.

ability to maintain their increased hardness for prolonged periods of time at temperatures up to 600 C. Or. stated in another way, they do not overage" upon heating to 600 C. (1112 F.). Even when heated to 600 C. for

contain as a ternary addition element from about 1% to about-3.5% by Weight of titanium havebeen found to be capable of precipitation hardening when subjected to heat treatments in accordance with the process of the inven;

don as disclosed above. These treated alloys have the 75 as specifically disclosed. A

'- After the alloy has been held at the required temperaover 100 hours the improved strength properties attained ture for a suflicient period to make it a single phase soluthrough the precipitation hardening process of the invention, the alloy is quenched or cooled rapidly to room temtio'n remained.

perature. In this condition the alloys are softest and The higher titanium alloys i.e. those having from about most easily formed and machined. 2.5 to about 3.5% by weight of titanium, also show im- After forming as desired the alloy is subjected to a proved strengths at temperatures up to about 800 C. second heat treatment or aging treatment. The pur- Typical room temperature tensile properties of an alloy pose of this aging treatment is to promote the precipitaprepared in accordance with the invention are disclosed in tion from the solid solution. The aging temperature is the following table to indicate the effects of the hardening closely interrelated with the alloy system in question. steps of the process of the invention.

Too high an aging temperature will promote what is called overaging. This means that the particles pre- Table II crpttatmg from the solution grow too large and are meffective in producing increased hardness. For the alloy A H d P t T of this invention the cr1t1cal temperature range has been compogflon Heat Treatment 22 gf zl s 22 found to be between about 500 C. to about 600 C. de- R0 tion 1 pending upon the particular proportions of the alloy.

As with the solution treatment the minimum time re- 1.0% Ti, 8% Al, A5 Rolled 1s 7 91.140 quired for aging" the alloy depends upon the section 8% 3 A1. L000; on 1% 10 15 81285 thickness and the mass of the material being heated and 1 0 g i p 29 2 m 420 therefore must be determined empirically. It is impor- '%i i .32? 32 f j 'tant, however, that the heating time be sufiic1ent to 1n- 500 O-.1 our. sure that the entire mass is heated to a temperature within the required temperature range to produce optimum g d g s c s g t o c e strength alloys. The constituent precipitating from the mm per Squaw solid solution should form its own normal atomic lattice arrangement different from that of the matrix. It should From Tahle n y be Seen that h y 111 the form fine discrete particles that remain coherent and cause rolled condltloh Possessed lhlfil'medlflte Strength a mutual distortion of the matrix lattice near the parand elongatlon l) fhlhlmum g fi ti'cles- This matrix distortion is the basis for the hardenmaximum 6101183000 the Sohlhofl treated 60114111011 ing process. It has been found, however, that the alloys as V2 s Water q is best' This leaves the prepared in accordance with the process of this invention alloy in a state of a m ductility for Subsequent room do not average upon heating to 600 for proonged etemperature forming operation. Then to promote maxisiods i.e. they do not lose their increased hardness even mum streng is performed- With the "aging when subjected to 600 C. for long periods. treatment the elongation value drops to 2%.

V The following table lists various alloy systems accord- From the. foreg it y be Seen that there s been ing to the invention which were given the s lutio and disclosed a low aluminum content iron ba e alloy system .aging treatments described abov The times i en which is sufiiciently ductile to permit working at room were optimum for the 0.1" thick specimens tested. temperatures and which when properly treated is capable Table I Alumlnum=.8%. Solution Heat Treatment I 7 Aging Heat Treatment Ir0n= Remainder v iMeit" No. Optimum Max.

' Percent Percent Temp.. Time. Quench Hardness. ging Time. Har

, T1 0 hours Medium B. T9 1. minut 1-..--.- o.5 900 1 Room Temp., 5.5 500 10 5.5

Water.

e 3.0 900 1 is 000 10 47 7' 3.5 900 1 15 600 20 40 0.8 0.05 900 1 a p .500 .10 9 1. a 0. 05 000 1 5. 5 500 10 17 1.8 0.05 900 1 7.5 500 10 a0 5 2. a 0. 05 900 1 10 570 10 as 2. s 0. 05 000 1 15 600 10 as. 5 3. 3 o. 05 900 1 1s. 5 000 10 42 From the foregoing it may be seen that iron base alloys of superior strength as well as high oxidation and corcontaining about 8% by weight of aluminum and which rosionresistance at elevated temperatures. There has also been disclosed a process for producing the alloy described.

Within'the scope of the appended claims it is understood that the invention may be practised otherwise than What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A structural material which is sufficiently ductile to permit working at room temperatures and capable on further heat treatment of high strength and oxidation resistance at elevated temperatures consisting essentially of a mixture of from about 8% to about 12% by weight of aluminum, from about 1% to about 3.5% by weight of titanium and the remainder iron, said mixture being a supersaturated solid solution and being further characterized by a Rockwell hardness of about 7 to about 20, and a tensile strength of about 80,000 pounds per square inch.

2. A structural precipitation hardened material capable of high strength and oxidation resistance at elevated temperatures consisting essentially of a mixture of from about 8% to about 12% by weight of aluminum, from about 1% to about 3.5% by weight of titanium and the remainder iron, said mixture being characterized by a Rockwell hardness of about 25 to about 47, and a tensile Strength of about 110,000 pounds per square inch.

3. The method of producing a low aluminum content iron base alloy material having sufiicient ductility to permit working at room temperatures and capable upon further heat treatment of high strength and oxidation re sistance at elevated temperatures by heating a mixture consisting essentially of from about 8% to about 12% by weight of aluminum, from about 1% to about 3.5% by weight of titanium and the remainder iron to a temperature between about 900 C. and 1000 C. until said mixture becomes a single phase solution and subsequently rapidly cooling said solution to produce a supersaturated solid solution.

4. The method of producing a low aluminum content iron base alloy material having high strength and oxidation resistance at elevated temperatures by heating a mixture consisting essentially of from about 8% to about 12% by weight of aluminum, from about 1% to about 3.5 by weight of titanium and the remainder iron to a temperature between about 900 C. and about 1000 C. until said mixture becomes a single phase solution, rapidly cooling said solution to produce a supersaturated solid solution and subsequently heating said solid solution to a temperature between about 500 C. and 600 C. to cause aging.

5. A precipitation-hardened article made of an alloy consisting essentially of about 23-12% aluminum by weight, about 1-35% titanium by weight, and the remainder iron.

6. A solution-treated article made of an alloy consisting essentially of about 8-12% aluminum by weight, about 13.5% titanium by weight, and the remainder iron, said article being characterized by sufiicient ductility to permit working at room temperatures and further characterized by high strength and oxidation resistance at high temperatures.

References Cited in the file of this patent UNITED STATES PATENTS 2,172,023 Gat Sept. 5, 1939 2,726,952 Morgan et al Dec. 13, 1955 2,768,915 Nachman et al. Oct. 30, 1956 2,859,143 Nachman et al. Nov. 4, 1958 OTHER REFERENCES Comstock: Titanium in Iron and Steel, 1955, pages 231-234, published by John Wiley and Sons, Inc., New York, NY. 

1. A STRUCTURAL MATERIAL WHICH IS SUFFICIENTLY DUCTILE TO PERMIT WORKING AT ROOM TEMPERATURES AND CAPABLE ON FURTHER HEAT TREATMENT OF HIGH STRENGTH AND OXIDATION RESISTANCE AT ELEVATED TEMPERATURES CONSISTING ESSENTIALLY OF A MIXTURE OF FROM ABOUT 8% TO ABOUT 12% BY WEIGHT OF ALUMINUM, FROM ABOUT 1% TO ABOUT 3.5% BY WEIGHT OF TITANIUM AND THE REMAINDER IRON, SAID MIXTURE BEING A SUPERSATURATED SOLID SOLUTION AND BEING FURTHER CHARACTERIZED BY A ROCKWELL HARDNESS OF ABOUT 7 TO ABOUT 20, AND A TENSILE STRENGTH OF ABOUT 80,000 POUNDS PER SQUARE INCH. 